Recent shortages in the production of oil have provided a revived interest for utilizing the large deposits of coal which are present in the United States. Coal has historically earned a bad reputation due to its highly polluting nature, not only from the standpoint of the ash which is produced, but also due to the production of NO.sub.x and SO.sub.x emissions from the sulphur and nitrogen contained within it. In order to alleviate and remove these polluting chemicals, efforts have been renewed in finding new processes and in designing burners which are capable of reducing the total amounts of pollutants which are produced. The environmental Protection Agency has also lowered the permissible amounts of the above mentioned pollutants which can be released to the atmosphere. It has thus been imperative to find new ways of combusting coal which will permit the reduction of the pollutants to the required levels.
During combustion, coal particles decompose to produce volatile fractions which include light gases, high molecular weight compounds, and solid materials referred to as char. Current methods of reducing nitrogen oxide emissions are generally directed toward creating conditions which favor the production of N.sub.2. This is primarily achieved by limiting the amount of oxygen available to the coal or other fossil fuel in the initial stages of combustion. Initially, less than the stoichiometric amount of air required to combust the coal is introduced with the coal to create a fuel rich condition. Greater amounts of air are then added, usually in one or two more stages. In the last stage, the combustion conditions often include using an excess of the stoichiometric amount of air which is required for combustion.
As used herein and in the appended claims, the term "stoichiometric" is meant to define the exact amount of a substance which will react in a specific chemical reaction with no excess of reactant or product.
Various types of burners have been utilized for purposes of creating the staged air conditions. Examples of such burners include circular wall fired burners and tangential non-swirling interacting flame burners. These burners are distinguished by the conditions of the heat release zone. In circular wall fired burners the heat release zone is close to the burner, while in the tangential non-swirling interacting flame systems, the heat release zone is spaced from the burner. These burners can be used in conjunction with various types of furnaces, for example: wall fired, arch fired, corner fired, and slot fired furnaces.
Circular wall fired burners include a fuel injector, a combustion air distribution system, some device to provide tangential velocity to one or more air streams, an exit refractory cone, and ignition and safety systems. In general, circular coal burners are typically high turbulence burners which produce high heat release intensities which favor the production of high nitrogen oxide levels.
Several wall fired burners have been designed to produce lower nitrogen oxide emissions than the standard circular coal burners. These burners operate by supplying controlled amounts of air in stages to the combustion zone.
An example of such a burner is one produced by Babcock and Wilcox. The burner includes a venturi/plug combination which delivers a homogeneous mixture of coal and primary air through a primary tube. Surrounding the primary tube are dual annular secondary air registers. The inner secondary air register contains a swirl vane to vary air swirl and control coal to air mixing. This burner is often called a dual register burner.
Another example of a circular wall fired burner is a Foster Wheeler low turbulence burner. In this burner, the primary air-coal stream issues from a tapered annular coal nozzle which is provided with an axially movable inner sleeve. By such provision, the primary air stream velocity can be varied at constant air flow. Such a burner is often termed in the industry as a dual throat burner. Air to the tubular passages is monitored by means of a master air register which operates by means of a pressure drop across a perforated plate which surround the air register. The amount of air to each of the passages is controlled by means of air register vanes.
The Steinmueller circular wall fired burner is characterized by a primary air fuel injector which is surrounded by a single secondary air passage and four outboard staging air ports. Swirl in the secondary air passage is controlled by varying the position of an annular fixed angle swirl vane assembly which is adjusted relative to a conversion cone. The outboard air ports are provided with air dampers for purposes of controlling flow to these ports.
The Environmental Protection Agency has presently targeted 1985 as the year for establishment of new source performance standards which are equivalent to 20% of those which exist today. Thus, the above described coal fired burners, while sufficient to operate under today's emission standards, will be inadequate to fulfill the projected goals. Newly designed burners and techniques are therefore necessary to comply with the projected standards.
Since approximately 75% of the nitrogen oxides arise from nitrogen contained in the coal, the primary emphasis is on control of the fuel NO formation. The oxidation of molecular nitrogen often called thermal NO, has been found to be affected by flame temperatures and combustion product quench rate. By minimizing the peak flame temperatures and maximizing the combustion product quench rates, the thermal NO formation can be reduced.
The formation of fuel NO is influenced primarily by the conditions in the heat release zone. Primarily, efforts are directed to burning under fuel rich conditions. Variations of the burner design can be utilized to control peak flame temperatures, product quench rates, and residence time in fuel rich zones. These are a function of the method of fuel and air injection, the distribution of axial and tangential velocity, the burner geometry and the like.
At the same time that the nitrogen oxides are desirably kept at a minimum, it is equally desirable to minimize the production of sulphur oxides.
Various methods have been tried for removing sulphur oxides from exhaust gases resulting from coal combustion. These methods are primarily directed to pretreatment of the coal to remove sulphur prior to combustion. Other treatments include passage of the combustion gases through lime slurries and the like prior to releasing the gases to the atmosphere. Other studies have investigated the injection of dry limestone into the stack gases.
In recent years, many investigators have explored the possibility of using sorbents for in situ sulphur removal from coal during combustion. Such experimentation proved that in situ removal of SO.sub.2 was feasible by means of the dry sorbents, but that the efficiency was not great enough to make the process economically attractive. Initial studies involved the injection of the dry sorbents into the combustion zone near the fuel injector. This was followed by monitoring the stack gases to establish SO.sub.2 levels.
Further studies indicated that the adsorption of sulphur from the coal constituted a very complicated chemical reaction. The efficiency appeared to be dependent on the stoichiometric ratio of the alkaline metal in the sorbent to coal sulphur, the physical characteristics of the sorbent including particle size, the combustion time-temperature history of the sorbent particle, and the type of gaseous sulphur compounds which are formed, i.e. reduced or oxidized. The last two factors, including the time-temperature history and the type of gaseous sulphur species can be controlled in part by the burner design.
It has now been found according to the invention that the sulphur which is contained in coal during combustion can be captured by a process of calcining the sorbent during combustion. By this process, the amount, distribution, angle, and turbulence of air which is provided to the coal during combustion is controlled. Under these same process conditions, the NO.sub.x emissions can also be kept desirably low.
This invention is directed toward overcoming the deficiencies of the prior art by helping to decrease the NO.sub.x and SO.sub.x emissions through a combustion process utilizing a sorbent and controlled combustion conditions.
The process of the invention can be employed in conjunction with various burners and furnaces, which are capable of providing the process conditions. A burner which is preferred for use in the invention process is also provided herein.